A process for combined nitrification and anaerobic ammonium oxidation (anammox) of wastewater is known in the art and typically used for treatment of ammonium-rich wastewater streams at mesophilic temperatures, i.e. in the range of from 25 to 40° C. In this process, half of the ammonium is first oxidized by ammonia oxidizing bacteria to form nitrite according to the following reaction equation:4NH4++3O2→2NH4++2NO2−+4H++2H2O  (1)The resulting ammonium and nitrite are then converted to dinitrogen gas by anammox bacteria according to the following reaction equation:NH4++NO2−→N2+2H2O  (2)It is known to carry out both process steps in a single reactor wherein the ammonia oxidizing bacteria and the anammox bacteria form compact sludge granules having a core of anammox bacteria and an outer rim of ammonia oxidizing bacteria. Such process is for example described in J. R. Vázquez-Padin et al., Water Science & Technology (2011), p. 1282-1288.
Anammox bacteria have a slow grow rate and therefore, the combined nitrification and anaerobic ammonium oxidation of wastewater is mainly applied for wastewater streams having a temperature around 30° C. Moreover, at mesophilic temperatures, the maximum growth rate of ammonia oxidizing bacteria is larger than the maximum growth rate of nitrite oxidizing bacteria, thus avoiding the undesired oxidation of nitrite to nitrate.
It would, however, be advantageous if the combined nitrification and anaerobic ammonium oxidation of wastewater could be carried out at lower temperature, i.e. below 25° C., preferably between 10 and 20° C., since domestic wastewater is typically available at such lower temperatures. Combined nitrification and anaerobic ammonium oxidation of wastewater at low temperatures has been proposed. In J. R. Vázquez-Padin et al., Water Science & Technology (2011), p. 1282-1288, suitable conditions for combined nitrification and anaerobic ammonium oxidation of diluted wastewater streams at low temperatures in a two-units configuration and in a single-unit configuration has been studied. It was found that it is possible to carry out combined nitrification and anaerobic ammonium oxidation in a single reactor at 20° C. The reactor has a height/diameter ratio of 5.5. Under the process conditions applied compact sludge granules having a core of anammox bacteria and an outer rim of ammonia oxidizing bacteria are formed. The process disclosed in Vázquez-Padin et al. is operated in batch mode.
In WO2011/110905 is disclosed a process for biological purification of ammonium-containing wastewater in an aeration tank at a low oxygen concentration (less than 1.0 mg/L dissolved oxygen) at temperatures between 7 and 25° C., wherein surplus sludge formed in the deammonification of sludge water from a digester at a temperature above 25° C. is fed to the aeration tank to continuously augment the amount of anammox bacteria in the aeration tank. In the process of WO2011/110905, part of the sludge is recycled over the aeration tank without removal of non-granular sludge.
There is a need in the art for an improved process for combined nitrification and anaerobic ammonium oxidation at low temperatures that can be carried out at an industrial scale and for non-diluted wastewater streams and wherein no continuous augmentation with anammox bacteria is needed.